1. Field of the Invention
The present invention relates generally to a manufacturing method for membrane electrode assembly of fuel cells, and in particular to a method for formation of membrane electrode assembly by printing processes so as to eliminate swelling and cracking of the membrane electrode assembly.
2. The Related Art
Fuel cells are an electro-chemical device that makes use of electrochemical reaction between a fuel containing hydrogen and an oxidizer, such as oxygen contained in the surrounding air, to generate electrical power. The fuel cells are advantageous in low contamination, high efficiency and high power density. Thus, developments and researches are intensively devoted to the fuel cell field for exploitation of the utilization thereof.
A typical fuel cell stack is made up of a structure comprising several layers of which an intermediate layer is constituted by an ion exchange membrane that allow for transmission/penetration of ions and two catalyst layers are positioned on opposite sides of the ion exchange membrane to serve as anode and cathode in which chemical reactions are carried out. Two gas diffusion layers are formed on outside surfaces of the catalyst layers, which are commonly made of carbon paper or carbon cloth. Reactants diffuse through the gas diffusion layers to reach the anode and cathode catalyst layers and the reaction products of the anode and cathode are released by diffusion through the diffusion layers. Two guide plates, which are made up of carbon boards, metal boards or graphite fiber composite material boards are mounted on outside surfaces of the diffusion layers. Gas guide channels are defined in the diffusion layers adjacent the guide plates for guiding the flow of the reactants and reaction products of the anode and cathode.
FIG. 1 of the attached drawings shows a cross-sectional view of a typical ion exchange membrane fuel cell. The fuel cell, designated with reference numeral 1, comprises a membrane electrode assembly (MEA) 10 made up of an ion exchange membrane 11 sandwiched between an anode catalyst layer 12 and a cathode catalyst layer 13. The anode of the membrane electrode assembly 10 comprises an anode-side gas diffusion layer 2 and an anode-side guide plate 3. The cathode of the MEA 10 comprises a cathode-side gas diffusion layer 4 and a cathode-side guide plate 5.
Also referring to FIG. 2, a practical fuel cell stack 100 is made up of a plurality of fuel cells 1 to which an anode collector board 61, an anode terminal board 62, a cathode collector board 63 and a cathode terminal board 64 are mounted by means of fasteners and airtight sealing. The fuel cell stack 100 further comprises air inlet and outlet 71a, 71b defined in the anode terminal board 62 to supply air that contains oxygen for the chemical reaction of the fuel cell stack 100. Hydrogen inlet and outlet 72a, 72b are defined in the anode terminal board 62 for supply of hydrogen for the reaction of the fuel cell stack 100. Coolant inlet and outlet 73a, 73b are also defined in the anode terminal board 62 for maintaining proper operation temperature of the fuel cell stack 100.
The MEA is the most important component of the ion exchange membrane fuel cell. Uniform coating of the catalyst layers on the opposite sides of the ion exchange membrane 11 plays an important role in the performance of the fuel cell. The materials that make up the MEA are often brittle and of high costs and thus the arrangement of a manufacturing process for the MEA is one of the key issues of the fuel cell manufacturing.
However, heretofore the catalyst layers are formed by spraying and such a spraying operation often causes repeated coating on local areas of the catalyst layers, which not only unnecessarily extends the manufacturing cycle of the MEA, but also leads to non-uniform coatings of the catalysts. This in turn makes variation of the local thickness, leading to unstable coating quality. Some conventional techniques may overcome such problems but they are not suitable for atomization of mass production.
In addition, the ion exchange membrane, upon coating of the catalyst layers, absorbs solvent of the sprayed catalyst solution, which causes swelling of the MEA and eventually leads to cracking of the catalyst layers.
Taiwan Patent Publication No. 447160 teaches how to treat the ion exchange membrane with solvents in order to overcome the deformation of the MEA induced in the coating operation. The ion exchange membrane is soaked in solvents, such as alcohol-based solvents, to cause pre-swelling. The catalyst is then uniformly coated on the surfaces of the membrane, which does not swell when contacting the coating solution. The coating is then dried and the ion exchange membrane shrinks back to uniform thickness to provide a high quality MEA. The MEA so formed is thereafter sandwiched between two gas diffusion layers and is further subject to heating and pressing to complete the manufacturing cycle thereof.
Taiwan Patent Publication No. 529195 also discloses treating the ion exchange membrane with solvents, wherein swelling is induced on the membrane by being treated with alcohol-based solvents. Catalyst is then coated on the membrane, which is in turn sandwiched between two gas diffusion layers. Thereafter, the semi-product is subject to heating and pressing to complete the manufacturing of the MEA. The swelling is done in a two-phase manner, in which two alcohol-based solvents are employed to treat the membrane respectively. For example, the membrane is first soaked in a monohydric alcohol solvent of high volatility and then treated with a polyhydric alcohol solvent of low volatility. Examples of the monohydric alcohol solvent include methyl alcohol, ethyl alcohol, propyl alcohol and mixtures thereof and examples of the polyhydric alcohol solvent include ethylene glycol, propylene glycol, butylenes glycol, glycerol and mixtures thereof.
However, the conventional methods involve complicated processes of treatment with alcohols, which to some extents do not completely solve the problems caused by swelling. Further, maintaining good quality control of such treatments is difficult. Thus, industrial utilization of such conventional methods is limited.